The present invention relates to a polymerization process for manufacturing various grades of polystyrene, and more particularly involves methods and apparatus for reducing impurities in the recycle streams of polymerization processes.
In polymerization processes such as polystyrene polymerization for instance, a recycle stream is taken from the reactor product stream and returned to the initial entry point for the reactor. The recycle stream typically contains unreacted monomer, diluents, decomposition products formed from polymerization initiators, and other contaminants. Typically, trace contaminants are present at levels ranging from 10 up to about 500 parts per million (PPM).
In the continuous process of polymerizing styrene monomer into polystyrene and high impact polystyrene (HIPS), a common process of carrying out the polymerization is in reactor vessels, using polymerization initiators such as t-butyl peroxybenzoate, benzoyl peroxide, and other peroxides. One disadvantage of using such initiators is that a byproduct of the polymerization process using these initiators is acid decomposition by-products, such as benzoic acid, which acidic species then react with the initiator and, as a consequence, inhibit further polymerization, when recycled with unreacted monomer back into the polymerization reactors. consequence, inhibit further polymerization, when recycled with unreacted monomer back into the polymerization reactors.
One method of handling this problem is that disclosed in U.S. Pat. No. 4,857,587 to Sosa, et al, in which recycled unreacted monomer is passed through a recycle treatment vessel to remove acidic species. The vessel would normally contain an adsorbent material such as alumina or clay to remove a major portion of the acid component from the recycle stream.
Other methods and apparatus for reducing the effect of acid decomposition by-products in the recycle stream are disclosed in the two Sosa et al patents, U.S. Pat. Nos. 4,777,210 and 4,861,827. The '210 patent discloses a preinversion reactor to control particle size and the '827 patent teaches the use of initiators which do not decompose into acid by-products during the polymerization process.
Sosa et al U.S. Pat. No. 5,200,476 issued Apr. 6, 1993, discloses a system for reducing volatiles in a polymerization line, said system utilizing partial condensers, total condensers, devolatilizers, and filter beds arranged in series.
Additional patents to Sosa, et al, include U.S. Pat. No. 5,527,511; and U.S. Pat. No. 5,531,967 wherein methods and apparatus are disclosed for removing undesirable volatile components having high boiling points from a product stream during polymerization of monovinyl aromatic compounds such as styrene, which methods include the use of heat exchangers, devolatilizers, and filter beds.
U.S. Pat. No. 5,540,813 to Sosa et al, issued Jul. 30, 1996, discloses methods and apparatus for purifying polymerized monovinyl aromatic compounds such as polystyrene by reducing the residual monomer content of the finished polymer, said methods and apparatus including a combination of heat exchangers and devolatilizers used in conjunction with a monovinyl aromatic polymerization reactor system.
The entire disclosures of each of the aforementioned Sosa et al patents, U.S. Pat. No. 4,857,587; U.S. Pat. No. 4,777,210; U.S. Pat. No. 4,861,827; U.S. Pat. No. 5,200,476; U.S. Pat. No. 5,527,511; U.S. Pat. No. 5,531,967; and, U.S. Pat. No. 5,540,813 are hereby incorporated herein by reference.
The disadvantages of conventional methods such as alumna/clay filter beds is that the beds must be regenerated often, which necessitates a time delay and is expensive; and devolatilization introduces heat into the recycle streams which affects the free-radical processes by creating species that may act as inhibitors; for example, decomposition products of phenolic antioxidants.